In order to achieve extremely high integrity and reliability in a fly-by-wire primary flight control system, the system should be designed in such a way that the operation of a large central computer is not necessary for basic control surface positioning, gain control and airplane stability augmentation functions; instead, each autonomous subsystem (e.g. force-control stick flight controller, rudder pedal assembly, spoiler actuator, elevator actuator, etc.) has its own small computer processing capability via a RACU (Remote Acquisition and Control Unit). The RACU exists in the primary flight control system for providing: data encoding/decoding, data transmitting/receiving, data acquisition, actuator control and redundancy management functions. If most of the special flight control system functions are computed and performed locally near the control surface, then computation time requirements of a large central computer would be greatly reduced. Also, in the utilization of power actuators at the aerodynamic control surface, there is a real problem in designing an actuator that will provide both adequate hinge moments and an adequate bandwidth, in so far as response characteristics of the aerodynamic control surface are concerned.
For large passenger-carrying commercial airplanes, there is a problem in the proper design and sizing of hydraulic power actuators for primary aerodynamic control surfaces, e.g., ailerons, rudders and elevators. The present approach to designing these actuators appears to be fundamentally wrong, because the piston diameter or effective area is sized for the maximum hinge moments to be encountered in both moving the primary aerodynamic control surface through an angular range and for producing a predetermined angle-of-deflection rate. In addition, the tubing or lines carrying pressurized hydraulic fluid and the control valves are sized to permit a maximum flow of pressurized hydraulic fluid in order to produce both the greatest hinge moment likely to be required and the predetermined maximum angle-of-deflection rate of the aerodynamic control surface. However, at slow flight speed of an airplane, i.e., during take-off or landing operations, both a large angle-of-deflection range and a high angle-of-deflection rate of aerodynamic control surface movement are required. But, at slow flight speed, there is less dynamic pressure acting on the aerodynamic control surface in comparison to high speed flight; therefore, the hinge moments at slow speed are relatively low. However, at high speed flight of an airplane, the dynamic pressure acting on a deflected aerodynamic control surface is near maximum; and the hinge moments and the resolution of angle-of-deflection rate requirements are also near maximum; whereas, the angle-of-deflection range is relatively small under this condition. Therefore, in designing for this high speed flight operation, the hydraulic power actuators produced are large and heavy. However, this would not necessarily have to be the case if there were a gain control device that could be inserted into the load path between the power actuator and its controlled aerodynamic surface. Prior studies of this problem have indicated that the result would be too complex a mechanical device for operating in the manner required.
With the advent of an all-electric flight control system for an airplane and the proposed utilization of an electro-mechanical or electro-hydraulic power actuator, there is a real design problem; because a key factor in the implementation of an all-electric flight control system for an airplane is the successful development of a suitable high-performance electric motor to replace the presently used hydraulic actuator. The electrical industry has had several practical breakthroughs in technology that make possible the development of an all-electric airplane. In the power-generation and actuation field, rare-earth-cobalt magnets, which are many times more powerful than the strongest Alnico magnets, are allowing the development of permanent-magnet generators and motors that are far superior to existing production components and provide unique opportunities not previously possible. Samarium-cobalt motors, gearboxes, and motor controllers are presently being developed for use on military airplanes and missiles.